A Continuation of ⑨ SOULS. The story happens when Cirno decided not to return her elements to the Soul of Nature, and decided to keep them.
Thus, due to the imbalance caused by a Fairy turning into an elemental, Gensokyo and the Outside World were both doomed. Optimistically, Cirno and Daiyousei went over to search for any remnants of Gensokyo's denizens.